1. Field of the Invention
The present invention relates to a dot impact line printer and, particularly, to a technique for regulating a distance between a printing head and a printing sheet.
2. Description of Related Art
In a dot impact line printer, printing is performed by a printing head with needles. The needles impact a surface of a printing sheet, which is supported by a platen, through an ink ribbon so as to form dots in the printing sheet. The depth at which the needles penetrate the pointing sheet is determined by input character information. Printing depth and the ink contrast becomes deeper by making the stroke of the printing head needles longer and paler by making the stroke shorter. Therefore, for constant printing, it is necessary to make the stroke distance constant, that is, to maintain a constant distance between the printing head and the printing sheet. The distance between the printing head and a platen will be referred to as xe2x80x9cplaten gapxe2x80x9d, hereinafter.
FIG. 1 shows a construction of a main portion of a conventional dot impact line printer. As shown in FIG. 1, the distance between printing mechanism 19 and platen 3 is regulated by rotating ball screws 53 and 54 by platen driven portions 50 and 51. Guide rails 60 and 61 are used to move the platen 3 in parallel precisely. Further, in order to move platen 3 in parallel precisely, platen driving portions 50 and 51 are synchronized with each other by drive synchronizer 52.
FIGS. 2 and 3 show a procedure for detecting a thickness of a printing sheet of a conventional dot impact line printer. As shown in FIG. 2, it is possible to detect the printing sheet thickness by urging printing sheet thickness sensor 55 against sensor support 20 through printing sheet 40. As shown in FIG. 3, platen 3 is moved to a predetermined position according to the thickness of the printing sheet thus detected.
An automatic sheet thickness detection mechanism disclosed in, for example, Japanese Patent Application Laid-open No. H6-055784 comprises a platen, which is moved from an initial position toward the side of a printing mechanism by an elevator mechanism using a lead screw to regulate a platen gap, a sheet thickness detection sensor fixed to the platen to determine the platen gap, a detection rod engaged with the sheet thickness detection sensor and in contact with a printing sheet to move according to the thickness of the printing sheet and a home position sensor for detecting the initial position of the platen. The platen is moved in parallel toward the side of the printing mechanism until the sheet thickness detection sensor operated by the movement of the detection rod corresponding to the sheet thickness is returned to an idle state.
A sheet thickness regulation mechanism disclosed in, for example, Japanese Patent Application Laid-open No. H2-258380 comprises a manual switch for manually selecting between an automatic setting mode and a selective setting mode, a switch for selecting a platen gap suitable for special sheets having partially different sheet thickness and a memory for storing data corresponding to the appropriate platen gaps for the special sheets.
As shown in FIG. 1, it is necessary to synchronize platen driving portion 51 provided in one end of platen 3 with platen driving portion 51 provided in the other end thereof with high precision. Further, it is necessary that ball screws 53 and 54 have substantially the same operating characteristics and the regulations thereof must be performed frequently.
That is, in order to move the long platen in parallel precisely by the elevator mechanism using such as lead screw, a complicated guide structure and a synchronizing structure for synchronizing both ends of the platen are necessary. Further, a large amount of regulation is necessary even after an assembling of the mechanisms.
As shown in FIGS. 4 and 5, which show problems related to conventional sheet thickness detection, it si impossible to set appropriate platen gaps for sheets having partially different thicknesses. For example, if a label sheet having four rounded corners is different in thickness between a label sheet portion and a peeling portion thereof, sheet thickness detection sensor 55 detects the thickness of the peeling sheet portion as shown in FIG. 4. When the position of platen 3 is set according to the detection data as shown in FIG. 5, it is necessary, in order to maintain speed in printing on the label portion, to maintain a response frequency of the printing mechanism. The response frequency depends upon a specific vibration frequency of the printing mechanism, the platen gap and a rebound coefficient of the sheet, etc. The rebound coefficient depends upon the thickness of the printing sheet and the number of sheets to be printed, etc. It is usual that the rebound coefficient is inversely proportional to the thickness of the printing sheet and, with the same platen gap, the response frequency of the printing mechanism becomes lower when the thickness of the printing sheet is increased. Therefore, the platen gap [becomes] will not be appropriate for the printing speed. In such case, it is usual that an operator confirms the thickness of the printing sheet at a printing position and instructs a change of the thickness of the sheet, which is automatically detected by the printing device.
Further, such printing device is usually requested to maintain the response frequency in order to maintain the printing speed. Since the response frequency depends upon a specific vibration frequency of the printing mechanism, the platen gap and a rebound coefficient of the sheet, etc., and the rebound coefficient depends upon the thickness of the printing sheet and the number of sheets to be printed, etc., and since the rebound coefficient is inversely proportional to the thickness of the printing sheet and, with the same platen gap, the response frequency of the printing mechanism becomes lower when the thickness of the printing sheet increased, there is a limit to the printing speed. However, there is no particular prior art considering this problem.
An object of the present invention is to provide a printer having a platen gap setting mechanism with a simple construction and high precision. Another object of the present invention is to provide a printer capable of setting an appropriate platen gap corresponding to a printing sheet having partially different thicknesses.
According to the first aspect of the present invention, a printer includes a printing head, and a platen arranged with a gap with respect to the printing head. The platen is pivotally coupled to a support to move towards and away from a printing path, a detector for detecting a thickness of a printing sheet, and an actuator for pivoting the platen about an axis of the support as a function of the detected thickness of the printing sheet. The support has a shaft provided in parallel to the platen and a frame rotatably supported by the shaft at two points, for supporting a center shaft of the platen at two points. As a result, an angle of the platen with respect to the shaft of the frame is easily regulated.
That is, in the printer of the present invention, the platen is mounted on the shaft, which is rotatable together with the frame. Therefore, it is possible to regulate the platen gap by the rotation of the frame about that shaft.
Thus, it is possible to remove the complicated platen driving portions at both ends of the platen, the guide rails and the drive synchronization portion, which are indispensable in the conventional printer.
It is preferable that the printing sheet feeder preferably includes means for automatically moving the sheet position detected by the detector for detecting the thickness of the printing sheet to the printing position.
That is, the printer of the present invention detects the thickness of the printing sheet at a first printing position. Therefore, it is possible to set a correct platen gap even when a sheet having partially different thicknesses is used.
Further, a table recorded with the gaps to be set for the sheet thickness detected by the detector is preferably provided. That is, it is possible to relax the influence of the rebound coefficient of the printing sheet, which is reduced with an increase of the thickness of the sheet, on the response frequency to thereby maintain the printing speed and improve the printing performance of the printer, by controlling the operation of the printer on the basis of the detected thickness of the printing sheet with using, for example, a gap table recorded with the air gap between a surface of the printing sheet and the printing mechanism, which is reduced with an increase of the thickness of the printing sheet, and provided in the memory.
The detector may be supported by the frame. That is, the construction of the printer can be further simplified by constructing the sheet thickness detector such that it is supported by the same frame as that supporting the platen.
According to the second aspect of the present invention, it is provided a printer which is characterized by comprising means for adjusting and controlling the platen gap, the gap between the printing head and the surface of printing sheet, responding to temperature variation of the platen. That is, the printer of the present invention is characterized by including a printing head, a platen arranged with a gap with respect to the printing head, means for feeding printing sheets into a gap between the printing head and the platen, means for detecting the temperature of the platen and means for adjusting said distance automatically by a predetermined value xc2x1xcex94d for reducing changes of said distance corresponding to a temperature information output from the detector.
In recent years, as it is required for a dot impact line printer to print faster, it is increased the number of needles arranged vertically. That is, it is developed a printer which has a vertically longer printing head to print a plurality of lines simultaneously. Consequently, it makes the diameter of a platen lager. Although a platen is made by a cylindrical substrate of light alloy such as aluminum with a hard lubber material pasted on the substrate, the lager the diameter of the platen, the greater the rotation driving moment becomes. Therefore, an alloy is used which is as light as possible in weight for its substrate and the cylinder is made thinner.
The other side, as heat capacity becomes smaller because thickness of the platen cylinder is made thinner, temperature variation of the platen becomes larger if it is repeated to print, to pause, etc. Responding to this temperature variation, heat expansion and shrinking of the metal material causes the diameter of the platen to change and the platen gap to change. In selecting the material of the platen substrate, it has been considered to make the material light in weight or hard. However, it has not been considered to select a metal material which makes the heat expansion coefficient of the platen substrate small.
As described above, the platen gap is important for printing quality. If the gap varies on printing execution, printing thickness changes. On a practical printer in which the change of the platen gap is large, an ink ribbon may contact with the printing sheet surface to stain it. In the first aspect of the present invention, the platen gap can be automatically controlled to become constant precisely by detecting the platen gap using a simple mechanism.
The second aspect of the invention is achieved on the above background to provide a low cost printer in which printing quality does not change due to temperature variation of the platen. In the printer of the second aspect of the invention, by forming the platen thin, thermal capacity of the platen becomes small so that a constant printing quality can be maintained if it becomes greater for the platen shape to be influenced by temperature variation. According to the second aspect of the invention, the degree of freedom of materials which may be selected is increased. In a printer which also comprises a mechanism to adjust the platen gap responding to the thickness of the printing sheet used, if it is changed to design, printing quality does not change due to temperature variation. Printing quality does not change even if the temperature of the platen varies during printing execution. Changes in a line are hardly visible by controlling the platen gap corresponding to the temperature variation. Even if printing is continued or interrupted, printing quality does not change and the ink ribbon does not contact the printing sheet surface to stain the print finish.
The printer according to the second aspect of the invention does not detect the platen gap directly during printing execution, but detects the platen temperature by a relatively low price temperature-electricity converter such as a thermistor, to record the value xc2x1xcex94d that should be adjusted into the standard value of the platen gap corresponding to the temperature information output from the temperature-electricity converter for each product or sample, and to control the platen gap by using the recorded value and the temperature information. It is not also a closed-loop automatic control which detects the gap on every adjusting and controls is always constant. By the second aspect of the present invention, printing quality can be controlled constant enough for practical use.
By the second aspect of the invention, a dot impact line printer is provided which can maintain printing quality constant enough for practical use at a low cost without providing any high cost means for detecting the platen gap by non-contact means or for controlling automatically by a closed loop.
The present arrangement can be embodied in prior printer which comprises means for regulating the platen gap corresponding to the thickness of the printing sheet used, with a small change of where the means for detecting the thickness of the printing sheet and means for regulating the platen gap automatically by using a detected thickness information of the printing sheet as well as said temperature information are provided.
Said means for regulating the platen gap may include a thickness table having previously stored gaps to set for the thickness information of the printing sheet, a temperature table having previously stored gaps to change for the temperature information, first control means for regulate the gap by receiving thickness information of the supplied sheet and referring to the thickness table and second control means for adjusting the gap by retrieving the temperature information on printing execution and referring to the temperature table.
This arrangement can be realized by adding hardware means for detecting the temperature information and supplying it to a control portion on the printer which can regulate the platen gap for the thickness of the printing sheet with adding no hardware and with changing software for controlling.
The second control means is preferably set to adjust the platen gap on printing execution. By such an arrangement, if the platen gap is changed during printing execution, no irregularity of printing is visible on the printed sheet.
The second control means can make the printing finish natural by executing adjustment for every gradual width change within a degree that a change of printing quality can be recognized.